1. Field of the Invention
The present invention relates to an optical module, and more particularly, to an optical module for differential quadrature phase shift keying (DQPSK), differential phase shift keying (DPSK), quadrature phase shift keying (QPSK), dual polarization quadrature phase shift keying (DP-QPSK), and the like.
2. Description of the Related Art
An optical communication network is constructed of optical fibers each serving as a medium for propagating an optical signal, and optical transceivers for transmitting and receiving the optical signal. The optical transceiver incorporates, in a housing thereof, an optical module for converting an electric signal into an optical signal and converting an optical signal into an electric signal, and a printed circuit board having control electronic devices, electric connectors, and the like mounted thereon.
A general optical module includes optical devices for performing opto-electric conversion, such as a laser (light emitting device) and a photodiode (light receiving device), which are mounted in a package thereof. The package is made of metal, ceramics, or the like in many cases. The package that is frequently used has a box-like shape, which provides an advantage that a mounting process is simple. Under a state in which a lid of the box-like package is not fixed, a substrate, an IC, optical components such as lenses, a light emitting device, a light receiving device, and the like can be arranged in plane inside the package. After the components are mounted inside, the lid is fixed to construct the package.
Heretofore, in the general optical module, the light emitting device converts a modulated electric signal into a light intensity-modulated signal to propagate an optical signal, and further, the light receiving device performs opto-electric conversion on the light intensity change to obtain an electric intensity change. In this manner, the signal is transmitted.
In recent years, however, along with an increase in transmission capacity of the optical module, so-called phase shift keying (PSK) has emerged as a method of propagating signals obtained by modulating a phase of light. This method is known as, for example, differential quadrature phase shift keying (DQPSK), differential phase shift keying (DPSK), quadrature phase shift keying (QPSK), dual polarization quadrature phase shift keying (DP-QPSK), and polarization multiplexed quadrature phase shift keying (PM-QPSK). In recent years, an optical transmission module, an optical reception module, a transceiver, and the like which are compatible with each method have been reported in an academic meeting and the like. In such an optical module using the phase modulation method, it is necessary to house a plurality of optical devices in the housing of the optical module, and mount optical components for multiplexing and demultiplexing optical signals. In particular, in the optical reception module, it is necessary to construct an interference optical system in the housing of the module so that phase-modulated optical signals are caused to interfere with each other to be converted into light intensity signals. Further, it is necessary to convert the optical signals into electric signals by the light receiving device.
By the way, at the same time, the optical transceiver has been required to be reduced in size and height in recent years, and therefore studies have been conducted to reduce the size and height of the optical transmission module and the optical reception module. As a method therefor, the recent optical module has included the optical devices and the optical components for multiplexing and demultiplexing optical signals, which are all mounted in a single housing.
Japanese Patent Application Laid-open No. 2005-309370 discloses an example of such an optical module. The optical module disclosed in Japanese Patent Application Laid-open No. 2005-309370 is an optical module using the intensity modulation method. The conventional optical module includes a plurality of light emitting devices (laser diodes) and a plurality of light receiving devices (photodiodes), which are fixed to a housing thereof. A total reflection wavelength filter and a wavelength separation filter are spatially arranged inside the housing so that the optical signals are reflected and allowed to pass through the filter, to thereby multiplex and demultiplex the optical signals. In the conventional example, the use of one optical fiber allows transmission of one transmission optical signal and two reception optical signals.
Further, according to the conventional example, an optical signal emitted from the light emitting device is condensed by the lens, and the optical signal is propagated to a capillary having a fiber integrated therein. Then, the optical signal exiting from the capillary is collimated by a collimator lens, and the collimated light is propagated in a space inside the housing. By mounting the capillary having a fiber integrated therein, it is possible to correct an optical axis by the fiber integrated in the capillary even if the light from the light emitting device is tilted relative to the optical axis.
In the conventional example, it is necessary to arrange in line the components within a range of from the light emitting device (laser diode) to the fiber collimator. Specifically, it is necessary to position in line the optical axes of the light emitting device, the condenser lens, the capillary, the collimator lens, the filters, and the fiber collimator. However, the above-mentioned components differ in shape and size, and in particular, the fiber collimator is mounted on a wall of the housing whereas the collimator lens and the capillary are mounted on a bottom surface of the housing. As a result, the conventional optical transmission module in which the optical axes of all the components are to be aligned in line has a disadvantage that the thickness of the housing cannot be reduced.
Similarly, in the conventional example, it is necessary to arrange in line the components within a range of from the light emitting device (laser diode) to a member having a reflecting function (total reflection wavelength filter). Specifically, it is necessary to position in line the light emitting device, the condenser lens, the capillary, the collimator, and the total reflection wavelength filter. Meanwhile, the positions and orientations of the fiber collimator and the wavelength separation filter are determined in advance, and hence there is almost no degree of freedom in designing. Further, the fiber collimator is fixed at a position that is regulated by the optical transceiver, and hence there is a disadvantage that when a spatial optical system of the conventional example is actually assembled, the size thereof increases. The size of the spatial optical system is proportional to the size of the housing. Therefore, the conventional optical module has a disadvantage that the housing is large.